1. Technical field of the Invention
The present invention relates to an apparatus and method for processing micro-V grooves for manufacturing immersion gratings.
2. Prior Art
When a large astronomical telescope is used to observe the motion of molecules existing in a low-temperature dark nebula, for instance, the telescope must have a resolution r=xcex/xcex94xcex=200 thousand for the 10 xcexcm wavelength band. FIG. 1 shows the configuration of a mid-range infrared high dispersion spectrograph (IRHS) which has a resolution such as that described above. In FIG. 1, the IRHS analyzes infrared rays sent from a pre-optical system (camera), using a collimator-cum relay optical system, and observes the analyzed spectra using a collimator-cum camera. The collimator-cum relay optical system is composed of an incidence slit, a reflecting concave mirror, and an immersion grating, and in particular, the immersion grating reflects and analyzes the rays.
FIGS. 2A, 2B and 2C show the principles of the immersion grating; FIG. 2A illustrates a reflecting vertical surfaces of the V-grooves in FIG. 3B are coated with metal by vapor deposition and work as reflecting surfaces, so they must be finished so as to be precisely parallel to the incident surface, and have a mirror surface finish.
However, these fine V-grooves have been produced conventionally by, for example, laser abrasion. Consequently, the materials which could be processed were limited to easily machinable materials such as silicon, quartz, etc., and hard, brittle materials (refractory materials) such as germanium and gallium arsenide cannot substantially be machined by abrasion. In addition, the shape of the grooves cannot be machined precisely by the laser abrasion method, and the processed surface cannot be finished to give a mirror surface. Consequently, the above-mentioned immersion grating essentially cannot be produced using a hard, brittle material according to conventional methods.
Another conventional method of grinding, for example that of using a grindstone has problems due to the clogging or wear of the grindstone, and the shape of the grooves cannot be precisely maintained and also the bottoms of the grooves are circular arcs in shape, so essentially the grooves do not have the required reflecting surfaces. diffraction grating, FIG. 2B is a sketch of a transparent grism, and FIG. 2C shows a reflecting immersion grating. The immersion grating, as shown in FIG. 2C, is a reflecting diffraction grating with an optical path filled with a transparent medium, and its angular dispersion, that is, the optical path difference xcex94L is given by 2nsL and is proportional to the refractive index of the medium. Therefore, its resolution r=xcex/xcex94xcex is given by 2L/xcex=2d tan xcex8/xcex . . . (1)
Immersion gratings such as those described above are disclosed in xe2x80x9cAn Immersion Grating for an Astronomical Spectrographxe2x80x9d (HANS DEKKER), xe2x80x9cImmersion grating for infrared astronomyxe2x80x9d (APPLIED OPTICS, Vol. 32, No. 7, March 1993), etc.
Materials used for the aforementioned immersion gratings include germanium (Ge), gallium arsenide (GaAs), lithium niobate (LiNbO3), and other optical elements suitable for infrared rays. These materials can transmit infrared rays with large refractive indices, although they are opaque to visible light. However, because these materials are hard and brittle, there is a problem that it is very difficult to machine the fine V-grooves.
More explicitly, as shown in FIGS. 3A and 3B, it is necessary to produce V-grooves as small as about 90 xcexcm high and 233 xcexcm wide accurately with a pitch of 4 grooves per millimeter on the grating surface of germanium or gallium arsenide, for instance, to achieve a resolution of 200 thousand in the 10 xcexcm wavelength band. In addition, the
The present invention is aimed at solving these problems. In other words, an object of the present invention is to provide an apparatus and a method for processing micro-V grooves for an immersion grating with a high resolution, on a hard brittle material such as germanium, gallium arsenide and lithium niobate.
According to the present invention, a micro-V groove processing apparatus is provided and composed of an ELID grinding device (4) with a cylindrical cutting grindstone (2) that rotates about a perpendicular axis Y, and a rotary truing device (8) with a cylindrical truing grindstone (6) that rotates about a horizontal axis X; the aforementioned cutting grindstone (2) is provided with extremely fine grinding grains and a vertical outer periphery (2a) and a horizontal lower surface (2b) that grind the workpiece (1); the abovementioned rotary truing device (8) forms the shape of the outer periphery and the lower surface of the grindstone by plasma-discharge truing and mechanical truing.
The present invention also provides a micro-V groove processing method wherein a voltage is applied between the cylindrical cutting grindstone (2) that rotates about the vertical axis Y and the cylindrical truing grindstone (6) that rotates about the horizontal axis X, thus by means of the plasma discharge, the shape of the vertical outer periphery (2a) and the horizontal lower surface (2b) of the grindstone are trued. Next the cutting grindstone (2) is mechanically trued by the truing grindstone (6) without applying a voltage, and while the surface of the trued grindstone is in contact with the workpiece (1) to form the micro-V grooves its outer periphery is dressed electrolytically.
According to a preferred embodiment of the present invention, the aforementioned plasma-discharge truing and mechanical truing can keep the radius of curvature of the circular edge between the vertical outer periphery (2a) and the horizontal lower surface (2b) of the grindstone less than 20 xcexcm.
Using the above-mentioned apparatus and method according to the present invention, the rotary truing device (8) maintains the shape of the outer periphery and the lower surface of cutting grindstone (2) by means of both plasma-discharge truing and mechanical truing, and can keep the shape of the circular edge between the vertical outer periphery (2a) and the horizontal lower surface (2b) of the cutting grindstone to a radius of curvature of 20 xcexcm or less. As a result, by using the cylindrical cutting grindstone (2) with extremely fine grinding grains formed in this way, the workpiece is ground by the cutting grindstone and is at the same time dressed electrolytically. So the workpiece can be ground to produce very excellent processed surfaces without having the grindstone becoming clogged, with the surfaces having a finish as good as a mirror. Therefore, an immersion grating with a high resolution can be manufactured using a hard brittle material such as germanium, gallium arsenide and lithium niobate.
The above-mentioned cutting grindstone (2) is a metal-bonded diamond grindstone using diamond grinding grains with a mean grain diameter of 1 xcexcm or less, and the aforementioned truing grindstone (6) is a metal-bonded diamond grindstone with diamond grinding grains.
This configuration allows the cutting grindstone (2) to be dressed electrolytically and to be trued by plasma discharge by the truing grindstone, and in addition, the cutting grindstone (2) can be trued mechanically by the truing grindstone (6).
The discharge voltage power supply (10) is provided to apply a voltage between the above-mentioned cutting grindstone (2) and the truing grindstone (6) to produce a plasma discharge.
The cutting grindstone (2) is connected to the positive terminal of the above-mentioned power supply, and the truing grindstone (6) to the negative terminal thereof, and voltage pulses are applied between the grindstones to produce a plasma discharge, thereby the cutting grindstone (2) can be trued with the truing grindstone (6) by the plasma discharge.